The disclosure pertains to charged-particle-beam (CPB) optical systems and CPB pattern-transfer apparatus as used in, e.g., CPB microlithography for manufacture of semiconductor devices such as integrated circuits and displays.
Charged-particle-beam (CPB) pattern-transfer apparatus have been developed to provide high-resolution pattern transfer for the manufacture of integrated circuits. CPB pattern-transfer apparatus offer higher resolutions than conventional visible-light or ultraviolet-light pattern-transfer apparatus. FIG. 4A illustrates a portion of a conventional CPB optical system for CPB pattern transfer. The system of FIG. 4A employs an electron beam EB as a charged particle beam. The electron beam EB is collimated upstream of a reticle 10 by a condenser lens (not shown). The collimated electron beam is directed to a region of the reticle 10 by field-of-view-selecting deflectors (not shown). The reticle 10 defines a pattern (usually a circuit pattern) to be transferred to a wafer 20 or other substrate termed generally a xe2x80x9cwaferxe2x80x9d herein. A first projection lens 30 and a second projection lens 40 form a-an image of the illuminated region of the reticle pattern on the wafer 20.
The projection lenses 30, 40 are magnetic lenses and comprise respective coils 45, 46 and yokes (magnetic pole pieces) 45a, 46a that are symmetric about an optical axis AX. The yokes 45a, 46a are made of iron or other magnetic material and have respective bores G1, G2 and G3, G4. The projection lenses 30, 40 are described with reference to an axial coordinate Z extending along the axis AX. The first projection lens 30 extends from an axial coordinate Z1 to an axial coordinate Z3, and the center of the projection lens 30 is at an axial coordinate Z2. The second projection lens 40 extends from an axial coordinate Z4 to an axial coordinate Z6, and the center of the projection lens 40 is at an axial coordinate Z5.
The coils 45, 46 of the projection lenses 30, 40, respectively, produce magnetic fields when energized by respective power supplies 47, 48. In some CPB optical systems, additional coils are provided inside the projection lenses 30, 40 to form supplemental magnetic fields for adjusting focal point, image rotation, and/or image magnification.
To increase throughput, CPB pattern-transfer apparatus are needed that expose larger wafer areas in each exposure while forming high resolution images. Improved magnetic lenses are important for high throughput CPB pattern-transfer.
The present invention provides, inter alia, CPB optical systems that exhibit lower aberrations. Such CPB optical systems comprise lens coils and yokes that provide a more abrupt decrease (i.e., less xe2x80x9cdroopxe2x80x9d) of the magnetic field at entrance and exit ends of the yoke. In specific embodiments, the more abrupt decrease of the magnetic field is produced without changing the yoke shape selected during the design of the lens.
Generally, according to the invention, coils for magnetic lenses are provided having an axis and a number of turns per unit length along the axis. The number of turns varies as a function of position along the axis. In a first representative embodiment, the coil has a first end and a central section, wherein the number of turns per unit length is greater in the first end than in the central section. Such a coil can have a second end in which the number of turns per unit length is greater than in the central section. In alternative embodiments, the coils are symmetric about the axis.
According to another representative embodiment, coils for magnetic lenses are provided, wherein an exemplary coil comprises an entrance subcoil situated on an axis. The coil has a number of turns per unit length of Nentrance. The coil also comprises a central subcoil, situated on the axis, that has a number of turns per unit length of Ncentral. The coil also comprises an exit subcoil, situated on the axis, that has a number of turns per unit length of Nexit. In specific embodiments, Nentrance is greater than Ncentral, or Nexit is greater than Ncentral, or both. At least one of the entrance subcoil, central subcoil, and exit subcoil can be symmetric about the axis.
According to another aspect of the invention, magnetic lenses are provided that include coils as described above and a yoke that surrounds the coil. According to an example embodiment, the magnetic lens comprises an entrance subcoil and a central subcoil. The subcoils are situated on an axis and have respective turns per unit length Nentrance and Ncentral. According to a specific example, Nentrance greater than Ncentral.
The magnetic lens can comprise a power supply for the entrance subcoil and a separate power supply for the central subcoil. The power supply for the entrance subcoil establishes a respective Ampere-turn value for the entrance subcoil and the power supply for the central subcoil establishes a respective Ampere-turn value for the central subcoil. The Ampere-turn value of the central subcoil is desirably less than the Ampere-turn value of the entrance subcoil. The magnetic lens can comprise an exit subcoil situated on the axis and having a turns per unit length of Nexit, wherein Nexit greater than Ncentral.
According to yet another aspect of the invention, CPB optical systems are provided. A representative embodiment of such a system comprises at least one magnetic lens having an entrance subcoil, an exit subcoil, and a central subcoil. Each of the coils has a respective number of turns per unit length. The subcoils are energized such that a product of the turns per unit length and the current in at least one of the entrance and exit end subcoils is greater than a product of the turns per unit length and current in the central subcoil.
A CPB optical system according to the invention can be configured as first and second projection lenses for use in a CPB pattern-transfer apparatus. At least one of the first and second projection lenses comprises respective entrance, exit, and central subcoils, each subcoil having a selected number of turns per unit length. The number of turns per unit length of at least one of the entrance or exit subcoils is selected to be greater than the turns per unit length of the corresponding central coil. The projection lenses can include one or more power supplies that energize the subcoils. The power supply energizing the entrance or exit subcoil provides a greater number of Ampere-turns than the power supply that energizes the central subcoil.
The foregoing and other features and advantages of the invention will become more apparent from the following detailed description of embodiments which proceeds with reference to the accompanying drawings.